Precooked foods and food preparations are the object of increasingly greater demand, but the production of such packages introduces problems, particularly on an industrial scale. A known process consists, after cooking, in placing the raw or pretreated foodstuffs in a bag, creating a sufficient vacuum in the bag and closing it by thermowelding. The vacuum must be of the order of 98%. There are currently used bags of plastic material having a thickness of about 90 microns. The process is carried out in a vacuum chamber including thermowelding means. Such a machine is currently considered "manual", because it requires an operator and the production speed is low.
On an industrial scale, the process is necessarily different and the difficulties are different. There is used a thermoforming machine comprising two rolls of plastic film unrolled one above the other. The lower film passes on a perforated recessed plate located in a vacuum chamber. The interior of the film is hollowed within the recess under the conjoint effect of temperature and underpressure existing at the bottom of the recess. Raw or pretreated foodstuffs are placed in the hollowed portion, then the upper film is deposited and thermowelded to the lower hollow film, so as to define a hermetically closed wrapping containing a quantity of foodstuff.
This wrapping method is difficult to practice when certain hot foodstuffs have a liquid or pasty consistency (for example sauces) and are in danger of being drawn outside the package in the course of formation, because of placing it under vacuum. In this case the vacuum obtained is not sufficient, and residual air is present.
French patent application 2 629 060 discloses a system of automatic closure for a package in which the foodstuffs can be cooked. This device comprises a simple valve permitting air, steam and gaseous products produced during cooking to leave freely in the course of this cooking; it theoretically avoids external air penetrating the packaging at the time of cooling. The device as disclosed is however too simple and does not permit achieving sufficient reliability for industrial use. Particularly, the valve is in danger of opening in the course of the period of storing, causing the packaging to lose its quality of a sterile container. Thus, a substantial portion of the production is in danger of losing the benefit of pasteurization in situ and certain food preparations can prove to be damaged upon opening the package.